Bone conduction hearing aid



Aug. 4, 1942. s, A, M A N v 2,291,942

BONE CONDUCTION HEARING AID Filed Oct. 17, 1940 2 Sheets-Sheet l INVEINTOR SAMUEL J. A. M. BAGNQ Y I v m ATTORNEYS A 1942- s. J. A. M. BAGNO 2,291,942

BONE CONDUCTION HEARING AID Filed Oct. 17, 1940 2 Sheets-Sheet 2 IVNVENTOR AMUEL J. A. M'. BAGNO ATTORNEYS Patented Aug. 4, 1942 BONE CONDUCTION HEARING AID Samuel J. A. M. Bagno, New York, N. Y., assignor to Kurman Electric 00., 1110., New York, N. Y., a corporation of New York Application October 1'7, 1940, Serial No. 361,518

' Claims.

This invention relates to hearing aids and more particularly to bone conduction hearing aids.

The primary object of my invention is to generally improve bone conduction hearing aids or audiphone receivers. Such receivers using a button for contact with the osseous structure of the head have been troubled by variable air gap or even complete freezing caused by change in position of the button when pressed against the head. This difiiculty has led to the use of bone conduction receivers of the inertia type. but these have disadvantages such as the need for a minimum mass for adequate response.

One primary object of the present invention is to avoid the production of a variable air gap in a button-type receiver. I have discovered that the button and the armature may be provided with adjacent surfaces which are in no way connected, except by a film of relatively viscous fluid material, in which case rapid movement such as the audio-frequency vibration of the armature will be transmitted to the button, but a slow change in position of the button, due to pressure against the head, will not be transmitted to the armature. The resulting bone conduction unit may be comparatively tiny in size and light in weight yet produces an excellent hearing response.

To the accomplishment of the foregoing and other more particularized objects which will hereinafter appear, my invention consists in the bone conduction receiver elements and their relation one to the other as are hereinafter more particularly described in the specification and sought to be defined in the claims. The specification is accompanied by drawings, in which:

Fig. 1 is a section through a bone conduction receiver embodying features of my invention;

Fig. 2 is a partially sectioned view taken in the plane of the line 2-2 of Fig. 1;

Fig. 3 is a section taken in the plane of the line 33 of Fig. 1;

Fig. 4 is a perspective View of the pole piece assembly;

Fig. 5 is a plan view illustrating an intermediate stage in the manufacture of the pole piece assembly;

Fig. .6 is a front elevation of the hearing aid mounted in a yoked head band;

Fi 7 shows the device in use and diagrammatically illustrates one possible supply circuit for the same;

Fig. 8 is a partially sectioned view schematically illustrating the parts of the receiver at rest;

Fig. 9 is a similar view showing a change in the relation of the parts of the receiver when applied to the head; and

Fig. 10 is a similar view with the parts set into vibration by audio-frequency current.

Referring to the drawings, and more particularly to Fig. '7, the bone conduction receiver l'2 may be applied at any convenient point to the osseous structure of the head, but is here shown in conventional location bearing against the mastoid bone immediately behind the ear. The receiver I2 is carried by means of a suitable bowed head band l4. It is supplied with audio-irequency current through thin flexible conductors 16 which may be twisted in accordance with customary practice but which, for the sake of clarity, are not twisted in the drawings. The receiver I2 is light in weight and tiny in dimension, ithaving a diameter approximately the same as the transverse or short dimension of the usual rectangular inertia type unit.

The receiver may be energized by conventional means such as a microphone l8 and battery 20, the output of which may be amplified by means of a microphone amplifier generally designated 22, and then supplied to receiver l2 through a volume control resistor 24. An amplifier of the vacuum tube type may also be employed.

Referring now to Fig. 1, the receiver l2 comprises a vibratile armature 30 responsive to an electromagnet assembly generally designated 32. A large area button 34 projects from one end of the case 36 and is adapted to bear against the head of the wearer. The button 34 is not mechanically connected directly to the armature 30. Instead these members are provided with surfaces which are closely adjacent but which have a definite and substantial clearance therebetween. The surfaces in question are connected solely by means of a film 38 of a highly viscous fluid material. The armature 30 is suitably supported as, for example, by means of a resilient diaphragm 40. The button 34 is independently supported as by means of a resilient diaphragm 42. With this arrangement any audio-frequency vibration of armature 3D is effectively and efficiently transmitted to the button 34, yet the air gap between the magnet poles 44 and the armature 30 remains unchanged, for any inward movement of button 34 caused by pressure against the head is not communicated to the armature 30.

The arrangement may be more clearly explained by reference to Figs. 8, 9, and 10 of the drawings. In Fig. 8 the parts are shown at rest, it being assumed that the unit has been removed from the head and is not in use. At this time the diaphragms 40 and 42 both assume their normal or unflexed position. In Fig. 9 it is assumed that the unit has been applied to the head but that there is no audi-frequency input. At this time the button 34 has been pushed inwardly by the force of the head band urging the button against the head of the wearer. This deflects the diaphragm 42 thus urging the plunger 46 further into the armature 30. The latter however is not displaced, and the diaphragm 40 remains unflexed. Thus the air gap 48 between armature 30 and poles 44 remains absolutely unchanged. The fluid material 38 is preferably selected to have very little or no static friction, and this eigplains whya change in position of the button 34 need not be accompanied by a change in position of the armature 30.

Referring now to Fig. 10, if an audio-frequency current is supplied to the coils 12 of the magnet assembly 32, the armature 30 is set into vibration at audio-frequency, and the resulting vibration is transmitted through the film 38 to the button 34. This results from the fact that the fluid material 38 is highly viscous and therefore offers a high mechanical impedance to rapid movement while offering little or no impedance to a slow movement.

The specific structural form of the invention here" disclosed may be described with reference to Figs. 1 through of the drawings. The case '36 is a drawn metal cup preferably made of al'ufninum.' It is closed by a cap or cover 54 which may also be made of aluminum, and which is threadedly received on case 36. Cap 50 carries the plunger diaphragm 42 and the armature diaphragm 40. These are spaced apart by a suitable spacer ring 52. The diaphragms and spacer ring are held within cap 50 by means of a diaphragm locking ring 54, said ring preferably being provided with holes 56 for a suitable key or wrench.

When the cap 50 with the assembly of parts carried thereby is screwed onto the case 36, the rotation of the cap may be employed to determine the air gap between the armature 30 and the magnet poles 44. The resulting adjustment is fixed by means of a cap-locking ring 58 which is turned in opposite direction against cap 50.

The button 34 is preferably made of Bakelite or similar synthetic resinous or plastic material. The center of the button is provided with a threaded projection 60, and the diaphragm 42 is slipped over projection 68 and is then locked in position by means of a threaded bushing 62 which may be referred to as a plunger. The plunger may also be made of Bakelite or like material.

The armature 30 is circular or annular in shape and is stepped or shouldered to receive the diaphragm 40. The parts are held together, as by spinning or turning a part 64 of the armature outwardly over the inner edge of the diaphragm. A thin annular washer 66 is preferably interposed between the diaphragm 40 and the spun flange 64. The diaphragm 4%! is preferably apertured as is indicated at B8 in order to afford free vibration, for otherwise the space between the diaphragms would be substantially sealed.

The magnet assembly is made up of the permeable pole pieces 44, a permanent magnet 10, and coils 12 surrounding the pole pieces 44. A small slit or air gap 14 is preferably provided between the pole pieces. Without this gap the permanent magnet 10 would be short-circuited by the permeable poles and there would be no sub.- stantial direct flux at the pole tips. On the other hand, a large gap would introduce too much reluctance in the alternating current magnet circuit. The small gap here indicated is a compromise for best all around efficiency.

The magnet assembly is mounted on a back piece 16 which is preferably made of a nonmagnetic material such as brass. The method of assembly may be explained with reference to Fig. 5. The back piece 16 is generally cruciform, the arms 18 being provided with threaded 'or tapped holes and the arms 82 having holes 84. A single piece of a suitable ferro-magnetic metal is used for the pole pieces. This is bent to U-shape and is secured to the back 16, as by hard soldering. The pole pieces are then cut apart, as by means of a suitable milling cutter which is run through the pole pieces as indicated by the broken lines at 14.

The coils 12 are separately wound. The permanent magnet bar 10 is inserted in the space between the pole pieces, following which the coils may be slipped over the free ends of the pole pieces. However, in the particular form of the invention here illustrated, provision is made for securing the leads running to the coils. For this purpose the arms 82 of the brass back piece are bent upwardly from the flat condition shown in Fig. 5 to the upright condition shown in Fig. 4. Small strips of insulation 88 (Fig. 4) are eyeleted to arms 82 as by means of eyelets 90 passing through the holes 84. The insulation strips 88 have additional holes receiving eyelets 92 which hold the conductors I6 running to the receiver unit. It will be understood that with the arrangement shown in Fig. 4 the permanent magnet 10 and the coils 12 may be added just as previously described. The coils are connected to one another in series and the free ends are led to the eyelets 92 and there soldered in position. The back piece 16 is rigidly secured to the bottom of case 36 as by means of small screws 96 (Fig. 1) passing through the case and threadedly received in the tapped holes 88.

The conductors l6 pass through a suitable hole 98 (Fig. 1) in casing 36. Additional holes Hi0 may be provided at diametrically opposite points on the casing to receive the pins or trunnions of a suitable yoked head band I4. One form of head band is shown in Fig. 6, the end I02 being frictionally pivotally secured to an angle piece m, which in turn carries a downwardly bowed yoke I06, the ends of which have inwardly projecting pins I08 which are received in the holes 1'90 (Fig. 3) previously referred to. In this way the receiver [2 is permitted to freely seat itself in flat position against the head.

The permeable pole pieces and the armature are preferably made of nickel alloy, but any suitable ferro-magnetic material may be employed. The permanent magnet bar inserted between the pole pieces is preferably made of aluminum steel alloy such as that known commercially as Alnic'o, although any suitable permanently magnetic material may be employed. The diaphragm's 40 and 42 are preferably made of-phosphor bronze.

The coils in the specific example here illustrated have an impedance of fifty ohms for both coils in series. Each coil is preferably made of two hundred and thirty turns of No. 35 enamelled wire. It may be mentioned however that the unit may be constructed as a high impedance unit for use directly in a vacuum tube output circuit without a transformer, and for this purpose I have employed coils having seventeen hundred turns of No. 44 wire resulting in an impedance of approximately one thousand ohms which, while a little less than optimum, is adequate because of the elimination of the transformer loss.

The gap M between the poles is preferably made quite small, say, /1000 of an inch, this being a compromise between an efficient permanent magnet, on the one hand, and an efficient A. C. magnet, on the other.

In the specific unit here illustrated the plunger has a diameter of approximately one-quarter inch and it fits within the armature for an axial distance of approximately one-sixteenth of an inch. The clearance between the plunger and the armature is two thousandths of an inch, that is, there is a difference of two thousandths of an inch in radius.

The connecting film in the present case is a belt grease called Cling Surface belt treatmen manufactured by Cling Surface Company of Buffalo, New York. This material is made in three viscosities termed light, medium, and heavy. I recommend the use of the heavy but have also used the medium with complete success. The heavy material has the advantage of bringing the response down to somewhat lower frequencies. The light material is also usable but is less desirable for reasons above indicated.

The unit of viscosity is the poise. It is a C. G. S. unit, and it defines the drag between two plates of unit cross-section. a unit distance apart, and immersed in the liquid. For the purpose of the present bone conductor unit, materials having viscosities of the order of one thousand poises are suitable.

The material used should be characterized by viscosity but not static friction. The distinction in question is readily tested, for the material here referred to may be poured into a pan, and while initially heaped, the material soon settles down to a perfectly level condition after a few minutes, thus showing that the heaped condition resulted solely from viscosity and not from any permanent static friction, for the latter would cause the material to retain its heaped shape. The reason it is desirable to have theoretically no static friction is that any considerable amount of static friction will tend to maintain a difference in the magnetic air gap.

While I recommend the belt grease specified above, a mixture which is satisfactory may be made by dissolving rosin in lubricating oil. Equal portions of rosin and lubricating oil may be used to form a very viscous fluid. Another substance that may be used is Venice turpentine.

The material used should also be permanent. Thus molasses or honey are common examples of materials having high viscosity and no static friction, and these materials may, in fact, be used for a time, but they are not satisfactory for practical permanent use because of changes resulting from dehydration, crystallization, etc. A mixture of glycol and molasses, or a mixture of glycerine and molasses, might be used, the hygroscopic action of the glycol or glycerine being relied upon to maintain the molasses in proper condition with constant viscosity. Ordinary petroleum jelly may be used, but this, while usable, is not as satisfactory as the belt grease referred to above. In the first place, its viscosity is relatively low, thus requiring a smaller clearance or tolerance between the relatively movable surfaces. Specifically, a clearance of only /1000 of an inch would be desirable. The smaller the tolerance the greater the cost of manufacture of the unit. In the second place, the petroleum jelly has some static friction.

A heavy grade of ordinary lubricating oil may be successfully employed, but in such case the tolerance becomes -even less and a close sliding fit of only, say, /1o,o00 of an inch clearance may prove desirable. While this is theoretically possible, it is not good for practical rapid production manufacture at moderate cost, because the parts must be made with excessive precision.

The operation of the device depends not only on the viscous material used and the clearance between the relatively movable surfaces, but also on the wetted area. The viscous drag varies substantially in direct proportion to the area, the relation being a linear one, except at very high efiiciencies. The term efficiency may be used to express the amplitude of the movement of the button relative to the amplitude of the movement of the armature. At low and moderate efficiencies the efficiency varies directly with an increase in wetted surface. However this rule does not apply at higher efiiciencies, say, about ninety per cent, for considerable increase in area may be needed to further increase the efiiciency.

The unit here illustrated has a transmission efficiency of about ninety per cent. This can readily be increased by lowering the mechanical tolerance or clearance, but inasmuch as the ninety per cent efiiciency is adequate, it is preferred to retain the large clearance for convenience in manufacturing the unit.

It will be noted that in the present unit the axialdimension of the wetted surface is relatively small (approximately one-sixteenth inch). In another experimental unit-I have employed a rod smaller in diameter, say, three-sixteenths of an inch, but with an axial dimension of one-quarter inch, and in that structure I obtained a transmission efliciency of about ninety per cent while using a very large clearance, specifically, 9 of an inch. The transmission efficiency varies inversely with the clearance, but the relation is probably not a linear one.

Inasmuch as the velocity of motion increases with frequency it might be anticipated that the transmission efficiency would be greater for high frequencies than for low frequencies. This is in fact the case, but no difficulty arises by reason of deficiency in low frequencies within the audiofrequency range, for the reason that the resistance offered by the head of the wearer is less for a low frequency vibration. As a matter of fact this characteristic of response falling away at low frequencies applies very directly and importantly to inertia reaction units and the present device is comparable or superior to inertia reaction receivers.

A similar consideration applies in respect to amplitude of vibration, or loudness. At high amplitudes there is a greater velocity of motion, and at low amplitudes a lower velocity of motion. The efiiciency of the unit falls away at lower amplitudes but at lower amplitude the resistance to vibration offered by'the head of the wearer is small (in proportion to the amplitude of vibration), and therefore the overall efiiciency of transfer of sound to the wearer is maintained. Here again the performance characteristic is analogous to that of an inertia unit when such a unit is subjected to a change in amplitude or loudness.

The unit here disclosed is quite small, it having a diameter of only three-quarters of an inch. The button occupies most of the end wall and is adequate in area for complete comfort to the user, it having a diameter of approximately three-fifths of an inch. The weight of the unit is less than that of comparable inertia units.

It is believed that the construction and principles underlying the operation of my improved bone conduction hearing aid, as well as the advantages thereof, will be apparent from the foregoing detailed description. It will also be apparent that while I have shown and described my invention in a preferred form many changes and modifications may be made in the structure disclosed without departing from the spirit of the invention as sought to be defined in the following claims.

I claim:

1. A bone conduction hearing aid comprising a case, vibratile driving means therein vibrated in response to audio frequency current, a button of large area projecting from the case for engaging the head, resilient means vibratably mounting said button on said case, a free slip con' nection between the button and the vibratile driving means, and a viscous fluid material at the slip connection between the button and said vi-' bratile driving means, whereby a slow change of position caused by pressure of the button against the head is not transmitted from the button to the vibratile driving means, but audio frequency vibration of the vibratile driving means is transmitted to the button.

2. A bone conduction hearing aid comprising a case, vibratile driving means therein vibrated in response to audio frequency current, a button means vibratably supporting said button independently of said armature, a free slip connection between said button and said armature, and a viscous fluid material at said slip connection, whereby a slow change of position caused by pressure of the button against the head is not transmitted from the button to the armature and does not change the air gap between the armature and the magnet but audio frequency vibration of the armature is transmitted to the button.

5. A bone conduction hearing aid comprising a case, a magnet fixedly mounted in said case, an armature vibratably mounted in said case adjacent said magnet, said armature being responsive to audio frequency current applied to the magnet, a button projecting from said case for contact with the head of the wearer, a free slip connection between said button and said armature, and a fluid material at said slip connection, said material being characterized by permanence, high viscosity, and low static friction, whereby a slow change of position caused by pressure of the button against the head is not transmitted from the button to the armature and does not change the air gap between the armature and the magnet, but audio frequency vibration of the armature is transmitted to the button.

6. A bone conduction hearing aid comprising a case, an electromagnet disposed in said case, a permeable armature of annular shape disposed at the end of said magnet, resilient means supporting said armature, a button projecting from of large area projecting from the case for engaging the head, resilient means vibratably mounting said button on said case, a free slip connection between the button and the vibratile driving means, and a fluid material at the slip connection between the button and said vibratable driving means, said material being characterized by permanence, high viscosity, and low static friction, whereby a slow change of position caused by pressure of the button against the head is not transmitted from the button to the vibratile driving means, but audio frequency vibration of the vibratile driving means is transmitted to the button.

3. A bone conduction hearing aid comprising a case, a magnet fixedly mounted in said case, an armature vibratably mounted in said case adjacent said magnet, said armature being responsive to audio frequency current applied to the magnet, a button projecting from said case for contact with the head of the wearer, a free slip connection between said button and said armature, and a viscous fluid material at said slip connection, whereby a slow change of position caused by pressure of the button against the head is not transmitted from the button to the armature and does not change the air gap between the armature and the magnet, but audio frequency vibration of the armature is transmitted to the button.

4. A bone conduction hearing aid comprising a case, a magnet fixedly mounted in said case, an armature vibratably mounted in said case adjacent said magnet, said armature being responsive to audio frequency current applied to the magnet, a button projecting from said case for contact with the head of the wearer, resilient the case, resilient means independently supporting said button, a cylindrical stud projecting from said button through said armature with a small clearance therebetween, and a viscous fluid material between said stud and said armature, whereby a slow change of position caused by pressure of the button against the head of the wearer is not transmitted from the button to the armature and does not change the air gap, whereas an audio frequency vibration of the armature is transmitted to the button.

7. A bone conduction hearing aid comprising a cylindrical case, an electromagnet disposed in said case, a permeable armature of annular shape disposed at the end of said magnet, an annular diaphragm made of resilient material connecting said armature to the case, a circular button projecting from and occupying substantially all of the area at one end of the case, an annular diaphragm made of resilient material connecting said button to said case, a cylindrical plunger projecting from said button through said armature with a small clearance therebetween, and a viscous fluid material between said plunger and said armature, whereby a slow change of position caused by pressure of the button against the head of the wearer is not transmitted from the button to the armature anfd does not change the air gap, whereas an audio frequency vibration of the armature is transmitted to the button.

8. A bone conduction hearing aid comprising a cylindrical case, permeable pole pieces disposed in said case in U formation with a small air gap at the back of the U, a permanent magnet bridging said air gap and polarizing said pole pieces, coils surrounding said pole pieces and adapted to receive an audio frequency current, a permeable ar-mature of annular shape disposed at the ends of said pole pieces, an annular diaphragm made of phosphor bronze or like resilient material connecting said armature to the case, a circular button projecting from and occupying substantially all of the area at one end of the case, an annular diaphragm made of phosphor bronze or like resilient material connecting said button to said case, a cylindrical plunger projecting from said button through said armature with a small clearance therebetween and a viscous fluid material between said plunger and said armature, whereby a slow change of position caused by pressure of the button against the head of the wearer is not transmitted from the button to the armature and does not change the air gap, whereas an audio frequency vibration of the armature is transmitted to the button.

9. A bone conduction hearing aid comprising a cylindrical case, permeable pole pieces disposed in said case in U formation with a small air gap at the back of the U, a permanent magnet vbridging said air gap and polarizing said .pole pieces, coils surrounding said pole pieces and adapted to receive an audio frequency current, a permeable armature of annular shape disposed at the ends of said pole pieces, an annular diaphragm made of phosphor bronze or like resilient material connecting said armature to the case, a circular button projecting from and occupying substantially all of the area at one end phor bronze or like resilient material connecting said button to said case, a cylindrical plunger projecting from said button through said armature with a small clearance therebetween, and a fluid material between said plunger and said armature, said material being characterized by permanence, high viscosity, and low static friction, whereby a slow change of position caused by pressure of the button against the head of the wearer is not transmitted from the button to the armature and does not change the air gap, whereas an audio frequency vibration of the armature is transmitted to the button.

10. In the operation of a bone conduction hearing aid comprising a magnet, an armature responsive to audio frequency current supplied to the magnet, and a button for transmitting the vibrations to the head of the wearer, the method of avoiding changes in the air gap between the armature and the magnet despite changes in pressure on the button, which includes connecting the armature to the button solely by means of a film of highly viscous fluid material. 7

SAMUEL J. A. M. BAGNO. 

